The present invention relates to a method of applying indicia to sheet-like materials, especially to a method of applying indicia to webs or strips of photographic paper. More particularly, the invention relates to improvements in a method of applying indicia to the rear sides of webs of photographic paper for the purpose of controlling the movements of such webs in copying machines wherein the paper is exposed to light passing through or reflected by originals and the thus obtained images are thereupon developed and fixed prior to severing of webs to form discrete prints or series of prints. The invention also relates to strips or webs which are provided with such indicia and to apparatus for detecting the indicia in a photographic copying machine or the like.
In accordance with presently prevailing techniques, indicia (hereinafter called marks for short) which are monitored to arrest webs of photographic paper immediately prior to severing are applied to the rear sides of webs during copying. Such procedure is not entirely satisfactory because the advancing mechanism for the web must be operated with an extremely high degree of accuracy in order to insure that the knife or knives will sever the web exactly across the center of a frame line between two neighboring images on the exposed and developed web.
Furthermore, it is now customary to coat photographic paper with a layer of water-repellent synthetic plastic material, such as polyethylene, in order to prevent the paper from absorbing excessive quantities of liquid during the transport through a developing machine. The application of marks onto the plastic layer at the rear side of photographic paper presents serious problems; in many instances, the marks cannot be readily discerned upon completion of developing treatment, i.e., they do not stand out sufficiently from the surrounding area at the rear side of the web so that they cannot be readily detected by available photoelectric or other monitoring means. The developing treatment involves contacting photographic paper with aggressive liquids which often attack the marks and render them illegible. Therefore, it happens again and again that the monitoring devices "overlook" a mark or that overly sensitive monitoring devices produce signals in response to detection of dust particles, faint blemishes or other imperfections which are not intended to cause stoppage of the web in a photographic copying or like machine.